1. Field of the Invention
The present invention relates to a communications system and method for communicating compression coded image data, audio data, and additional information via a transmission path such that a temporal sequence of the data can be correctly reconstructed even after a transmission path error occurs.
2. Description of the Prior Art
The development of communication systems using optical fibers and other high speed transmission paths has greatly increased communication capacity. This has spurred the development of digital communication systems, which are now used not only for digital computer data, but also for transmitting digitized image signals, audio signals, and related additional information.
Asynchronous transfer mode (ATM) systems, for example, can operate at speeds in excess of 155 megabits/second, and practical ATM systems for communicating digital data are now available.
Formal standards for ATM systems have been adopted by the International Telecommunication Unionxe2x80x94Telecommunication Standardization Sector (ITU-T) and the ATM Forum, for example, and numerous related publications have been issued.
Japanese Patent Laid-Open Publication 8-307859 (referred to below as prior art example 1), for example, teaches a means for transmitting image, audio, and related data coded for commercial digital VCR use over an ATM network. A method for allocating the moving picture data of this digital VCR data stream to ATM cells is disclosed in this prior art example 1.
More specifically, as shown in FIG. 10, an 80-byte DIF (digital interface) block is split into two 40-byte parts, and the remaining 8-byte part of the 48-byte ATM cell payload is used for e.g. error correction parity. Alternatively, a 4-byte portion of the remaining 8 bytes is used for error detection code. In another method of the above-noted prior art example 1, three 80-byte DIF blocks are joined to form a single 240-byte data block, which is then split into five 48-byte ATM cells.
A further method for writing moving picture data to ATM cells is a so-called xe2x80x9cMPEG over ATMxe2x80x9d method taught in xe2x80x9cVideo On Demand Specification 1.0, af-saa-0049.000xe2x80x9d, issued by The ATM Forum Technical Committee on Audiovisual Multimedia Services in December 1995 (referred to below as prior art example 2). Numerous descriptions of this specification have also been published. Basically, MPEG over ATM is a method for joining two continuous MPEG2 Single Program Transport Streams (STPS), each 188 octets long, into one 376 octet stream to which an 8-byte AAL 5 trailer is added, and then allocating this 384-byte stream to be split into eight ATM cells, each cell corresponding to a 48-octet payload, for data transmission.
A further another method for writing moving picture data to ATM cells is disclosed in U.S. Pat. No. 5,533,021 (referred to as prior art example 3, hereinafter). In this prior art example 3, a transport system (TS) packet stream sequence is split into a plurality of packets in the ATM adaptation layer and mapped into a cell size AAL-5 Protocol Data Units (PDUs). In this example, however, there is no suggestion or teaching of a feature of appending time information to every packet for identifying the location thereof.
These conventional methods are, however, involved with problems or drawbacks as to be described below.
Specifically, in these conventional methods, temporal information or time code data indicating when each payload was coded is not contained in each payload. As a result, when an error occurs in the communications network, the receiver is unable to determine the time of error recovery, resulting in disruption of reproduction of images associated with the payload in which the error occurred.
This problem is described more specifically with reference to accompanying FIG. 11.
The top row 111 in FIG. 11 represents three continuous frames of data to be transmitted where the frames are identified as frame n, frame n+1, and frame n+2. The places at which an error occurs during transmission over the communications network are indicated with hatching in rows 112, 113, and 114.
For example, when a transmission load increases to a point exceeding a processing capacity of an ATM switch on an ATM communications network, a problem of cell loss is caused. When cell loss occurs, an error in successive bursts as indicated in FIG. 11 occurs.
The error is confined within frame n+1 in row 112, but expands to frame n and n+1 in row 113, and moreover in row 114 starts in frame n, continues through frame n+1, and extends into frame n+2. When an error occurs as shown in FIG. 11 and the error is recovered, there is no means available for identifying at what point in these successive frames error recovery occurred. More specifically, if error recovery occurs in frame n+1 in row 112, frame n+1 in row 113, or frame n+2 in row 114, there is no method for determining to which frame the data received immediately after error recovery belongs. As a result, the data for frames n and n+1 in row 113 may be merged into one frame of data, and data for frames n and n+2 in row 114 may be merged. In addition, error recovery may occur in the same frame in which the error occurred in row 112, for example, but there is no means of determining that the data before and after error recovery can be processed as coming from the same frame because there is no means of detecting that the error and error recovery occurred in the same frame.
When the segment in which an error occurred is re-transmitted, it is necessary to determine where the error occurred. This error determination is, however, not possible with the method according to the prior art example 1. It is therefore necessary to re-transmit the entire data block. It should be noted that the above-described problems with prior art example 1 occur regardless of the length of the error.
The above-described problem in the conventional method is involved due to lack of the time code data or time information in each cell payload, for identifying where or to what period in the image signal stream the image information contained in a given cell payload belongs.
In the conventional method using MPEG2 encoding which is a combination of interframe predictive motion compensation coding and discrete cosine transform (DCT) coding, when an error in the code stream occurs as a result of an error on the communications network, the error is propagated to a macroblock coding unit. This means that, because of interframe predictive coding, the error is propagated along the time base.
The problem with MPEG2 coding likewise occurs due to the reason because relatively large macroblocks are used as the encoding unit and because interframe predictive coding is used.
With regard to the aforementioned problems, there is a need for a communication apparatus which allows to determine a time period or frame number in the image signal stream to which every successfully received packet belongs even when an error occurs in a communication network. There is a further need for a communication apparatus which surely prevents propagation of errors in the communication network.
To meet the above described needs, an essential object of the present invention is to provide a novel communications system for transmitting a stream of multimedia digital data over a distribution communications network via a transmission path by way of a transmitter.
The communications system of the present invention comprises: a data stream generator for coding image data, audio data, other associated additional information in a unit of an image data group and generating a continuous data stream thereof together with temporal information indicative of a temporal location of the image data group; a packetizer for segmenting the data stream into a plurality of data packets; a packet header adding device for adding the temporal information as a packet header to each data packet to thereby produce a transmission packet, wherein said temporal information is regularly updated based on a update cycle of the image data group; and a cell generator for dividing the transmission packet into a plurality of cells to be transmitted to the transmission path.
Another object of the present invention is to provide a novel method of transmitting a stream of multimedia digital data over a distribution communications network via a transmission path by way of a transmitter.
The method comprises the steps of: coding image data, audio data, other associated additional information in a unit of an image data group and generating a continuous data stream there of together with temporal information indicative of a temporal location of the image data group; segmenting the data stream into a plurality of data packets; adding the temporal information as a packet header to each data packet to thereby produce a transmission packet, wherein the temporal information is regularly updated based on a update cycle of the image data group; and dividing the transmission packet into a plural be transmitted to the transmission path.
As a result, a receiver can detect the temporal information, which is typically a time code, from each transmitted packet, and thereby determine the time at which a packet was encoded in the stream. Thus, the temporal information associated with data received after recovery of an error can be known immediately by the receiver even when an error occurs on the communications network over which a data stream containing image data, audio data, and other information is transmitted.